Serial bus communication protocols have long been utilized for communications between two devices. This serial communication can provide long range or short range communication between the two devices and can either be “synchronous” or “asynchronous.” For asynchronous transmission, there are provided two independent clocks, one at the master and one at the slave node (note that either device on either end of the communication path can be either the master or the slave) that are each operable to receive or transmit data based solely upon their clock. Asynchronous communication tends to be somewhat slower than synchronous communication since there will naturally be a finite error between the two clocks. For synchronous communication, either a separate clock signal is provided between the two devices on a separate clock line, or some type of clock recovery is utilized. One type of synchronous serial transmission protocol that utilizes a separate clock line is referred to as I2C. In a clock recovery system, the clock signal is overlapped with the data on the same line, such that the clock information can be recovered from data transitions. One type of such clock recovery protocol is Manchester coded PSK. Another type, that associated with the present disclosure, is Universal Serial Bus (USB).
In order to maintain sync between the two systems, the receiver will typically “lock” onto the received data and extract the clock information therefrom. There will typically be provided a receive clock, which will have the frequency and phase thereof varied to substantially equal the frequency and phase of a transmit clock which is extracted from the receive data. One technique for providing this receive clock and adjusting the frequency and phase thereof is a phase locked loop. For continuous transmission systems, such as Manchester coded PSK, data transmissions are present on a substantially continual base, such that the phase and frequency error between the receive clock and the transmit clock and be continually minimized or corrected for. However, with respect to the USB transmission system, these have what is referred to as “bursty” communications; that is, data is only present in bursts. Therefore, substantially continual data transitions are not present in order for a phase locked loop to lock onto. As such, during times of no transmission, the receive clock may drift in phase and frequency and, upon receipt of the next burst of data, lock will again have to be acquired before the integrity of the data reception can be guaranteed.